As communication technologies have advanced in recent years, various kinds of development have been conducted on an optical circuit which is part of an optical communication system. Since a planar lightwave circuit is excellent in terms of improving functions, downsizing and lowering costs, expectations are running high for an optical waveguide device.
In the optical waveguide device, in order to couple the optical waveguide with the optical fiber in a highly efficient manner, the mode of the optical waveguide needs to match the mode of the optical fiber. In general, compared with the optical fiber, the relative refractive index difference Δ of the optical waveguide is large, and the mode diameter of the optical waveguide is small. Here, using a core refractive index n1 and a clad refractive index n2, the relative refractive index difference Δ is represented as follows: Δ=(n1−n2)/n1. Moreover, when the light intensity distribution of guided-wave light can be approximated by Gaussian, the mode diameter is a diameter (MFD: Mode Field Diameter) of a portion where the light intensity becomes 1/e2 (e is the base of natural logarithm) with respect to a maximum value of the light intensity (which usually corresponds to the central portion of the core).
The general idea is to make the mode diameter of the optical fiber close to the mode diameter of the optical waveguide (see Patent Document 1, for example). However, changing the mode diameter of the optical fiber is difficult especially in the case of a multi-channel fiber array. Even if it is possible, costs are extremely high. Accordingly, in the past, only an optical waveguide pattern has been changed in order to realize low-loss coupling (see Patent Documents 2 to 8 and the like, for example).
However, Δ of the optical waveguide gradually increases as the optical waveguide device is downsized, and the processing size of the optical waveguide needs to be ultrafine and highly accurate according to the above method. Therefore, it is becoming difficult to adjust the mode diameter to that of the optical fiber.
Accordingly, one of the methods to enlarge the mode diameter by changing the structure of the optical waveguide is to form a new optical waveguide whose Δ is an intermediate value between those of the optical waveguide and the optical fiber such that the new optical waveguide is inserted between the optical waveguide and the optical fiber (see Patent Documents 9 to 12 and the like, for example).
Patent Document 1: Japanese Patent No. 3,318,406
Patent Document 2: JP-A-06-027334
Patent Document 3: Japanese Patent No. 3,663,310
Patent Document 4: JP-A-2000-235128
Patent Document 5: JP-A-2001-337252
Patent Document 6: JP-A-2002-258078
Patent Document 7: JP-A-2005-115017
Patent Document 8: JP-A-2006-146212
Patent Document 9: JP-A-05-249331
Patent Document 10: Japanese Patent No. 3450068
Patent Document 11: JP-A-04-015604
Patent Document 12: JP-A-03-081740